1. Field of the Invention
The invention relates to an intake system for an internal combustion engine, which includes an intake air flow control valve that is formed of a valve body that adjusts the flow passage area in an intake pipe and a valve shaft about which the valve body pivots.
2. Description of the Related Art
In an internal combustion engine, the combustion efficiency is improved by producing vortex flows in the intake air that is supplied to a combustion chamber. Generally, vortex flows are classified into a swirl flow, which is a lateral vortex flow that eddies and proceeds along the circumferential direction of a cylinder, and a tumble flow, which is a vertical vortex flow that eddies and proceeds along the axial direction of the cylinder.
There is an existing technology in which a tumble control valve (hereinafter, referred to as “TCV”) is provided in an intake pipe to produce a tumble flow in an intake air flow. In an internal combustion engine that is provided with a TCV, the flow passage area in an intake pipe is adjusted by controlling the TCV. The flow of the intake air that passes through a space around the TCV is changed based on the flow passage area, and the intake air flow is constricted by passing through the space around the TCV. If the constricted intake air flow enters a combustion chamber, a tumble flow is produced in the combustion chamber. For example, Japanese Patent Application Publication No. 2001-248450 (JP-A-2001-248450) describes a TCV that includes a valve body that is provided in an intake pipe and a valve shaft that extends perpendicularly to the center line of the intake pipe. The opening amount of the TCV is adjusted within an opening amount range from the first opening amount to the second opening amount. The flow passage area when the opening amount of the TCV is the first opening amount differs from the flow passage area when the opening amount of the TCV is the second opening amount. The intake air passes through a space between the valve body of the TCV and the wall of the intake pipe, and is constricted to the space side. The constricted intake air flow is introduced into a combustion chamber, and then hits the wall of the combustion chamber to produce a tumble flow in the combustion chamber. Because the gas in the combustion chamber is stratified by the tumble flow, efficient combustion takes place and the combustion efficiency is improved.
However, the orientation of the valve body of the TCV described in JP-A-2001-248450 is always against the flow of the intake air. Therefore, even if an actuator is driven in order to maintain the opening amount of the TCV, it is not possible to reliably maintain the opening amount. In addition, it is considered that the position of the valve body is always changed by the intake air flow and adjustment of the position of the valve body, which is made by the actuator. Accordingly, it is difficult to maintain the optimum opening amount. Such a problem may occur not only in the TCV described in JP-A-2001-248450 but also in any intake systems that include a TCV that has a valve body of which the opening amount is required to be maintained at the first opening amount or the second opening amount.